A fuel assembly in a nuclear reactor includes a matrix of parallel rods containing fissionable fuel and/or water coolant flow. These parallel rods are held at a fixed spacing by spacer meshes located intermittently along the length of the fuel assembly. The matrix of fuel rods is supported at their lower ends by a lower tie plate, which also guides the fuel rod end plugs laterally. The lower tie plate also includes flow holes to provide an inlet for coolant flow into the fuel assembly. Similarly, the top end of the rod matrix is covered by an upper tie plate that restrains the fuel rod upper end plugs laterally and provides flow holes for the exit of coolant from the fuel assembly. One or more of the water and/or fuel rods is used as a structural member that is attached to both the lower and upper tie plates and serves the purpose of carrying a structural load of the assembly and maintaining a fixed distance between the lower and upper tie plates.
Prior designs using the water rod as the structural member typically have complex assemblies including members to transition from a large to a smaller diameter and/or end fittings to attach to the lower and upper tie plates as illustrated in FIG. 1. Even when the water rod is not used as the structural member, the assembly is complex as shown in FIG. 2 with many of the same components.
In particular, referring to FIG. 1, a conventional water rod has a central cylindrical tube 10 secured between an upper tie plate 12 and a lower tie plate 14. The central tube 10 has a plurality of inlet flow holes 16 that allow water to be driven through the rod, thus introducing moderating material within the fuel rod matrix. An upper reducer 18 and a lower reducer 20 are welded to upper and lower end of the central tube 10 respectively. The reducers 18, 20 effectively reduce the diameter of the central tube 10 to accommodate end fittings. An upper end fitting 22 and a lower end fitting 24 are welded to the upper and lower reducer 18, 20, respectively and are configured to be secured to the upper and lower tie plate, respectively.
Referring to FIG. 2, when the water rod is not intended to be the structural member for the fuel assembly, even more intricate end fittings are coupled to the central tube for connection to the upper and lower tie plates.